1. Field of the Invention
The present invention relates to solid state drawing processes and oriented polymer compositions produced by solid state drawing processes.
2. Description of Related Art
Oriented polymer compositions are desirable for having higher strength and stiffness over non-oriented polymer compositions. Historically, polymeric films and fibers have enjoyed the benefits of orientation through drawing processes. However, when a polymer cross section becomes larger than that of a film or fiber, drawing to a controlled and consistent shape becomes more complex and new drawing processes are necessary.
Great Britain (GB) patent 1311885 discloses a solid state die drawing process to address the challenges of orienting larger cross section polymer compositions, which the patent identifies as compositions having a cross-sectional area of 0.01 square inches (6.45 square millimeters) or more or with all cross sectional dimensions greater than 0.05 inches (1.27 millimeters). The solid state die drawing process requires drawing a polymer composition billet through a lubricated drawing die in the polymer composition's solid phase at a temperature below the polymer composition's melting temperature (Tm). The drawing die forces the polymer composition to converge towards a specific shape, causing alignment of polymer chains. According to GB1311885, one of the challenges with large cross section billets is bringing the entire cross section to a uniform temperature prior to drawing in order to allow control of the extrusion velocity of the billet into the drawing die. Small cross section articles such as films and fibers do not present such a challenge.
Orientation of filled polymer compositions is of particular interest. Filler offers numerous benefits, perhaps the most recognized is reduction in raw material cost for the polymer composition. Use of wood fiber fillers in oriented polymer compositions has been of particular interest for fabricating oriented polymer compositions that serve as an alternative to wood decking materials (that is, composite decking). Organic fillers, however, are subject to handicaps including color bleaching when exposed to the sun, and to decomposition, mold and mildew when exposed to humidity even within a polymer composition. Inorganic fillers are attractive because they are not subject to these handicaps. However, inorganic fillers tend to have a higher density than organic fillers. Additionally, reactive inorganic fillers such as Portland cement and gypsum are reactive with water (see, for example, PCT publication WO 2004/009334), which can result in an unstable polymer composition density in humid environments.
Incorporating void volume in a filled oriented polymer composition reduces the composition's density. U.S. Pat. No. 5,474,722 ('722) discloses use of blowing agents with organic and mica fillers (see Examples 3 and 9 in '722) in order to reduce the density of an oriented polymer composition. Blowing agents expand to foam the polymer composition in order to establish void volume. Foamed compositions contain blowing agent. Foaming requires a foaming step and control of foaming agent in the process.
Cavitation is a desirable alternative for reducing an oriented polymer composition density without the use of a blowing agent. Cavitation induces void volume proximate to filler particles while drawing a polymer composition containing the filler particles. For example, European Patent 1242220B1 provides an example a polypropylene composition filled with wood filler (composition density of about one gram per cubic centimeter (g/cm3)) that is drawn at a drawing rate of 48 inches (122 centimeters) per minute to obtain an oriented polymer composition having a density of 0.59 g/cm3 centimeter. Drawing compositions containing up to 22 weight-percent of mica filler in polypropylene also reveal void volumes from cavitation of up to 28.5% and densities down to 0.76 g/cm3. (W. R. Newson and F. R. Maine, ORIENTED POLYPROPYLENE COMPOSITIONS MADE WITH MICA, handout from 8th International Conference on Woodfiber-Plastic Composites, Madison, Wis., May 23-25, 2005).
PCT publication WO 2004/009334 ('334) discloses cavitation during orientation of polymer filled with reactive inorganic fillers such as Portland cement. '334 discloses both die drawn and free draw processes. The lowest density '334 reveals for a die drawn oriented polymer composition is 0.82 g/cm3. Lower densities are reported for free drawn compositions by using a linear draw ratio of greater than eleven. However, free drawn oriented compositions having such a large linear draw ratio (greater than eleven) tend to suffer from a low delamination force. That is, they delaminate or fibrillate more easily along the drawing direction than free drawn compositions having a lower linear draw ratio, as well as die drawn compositions. Moreover, a free draw process offers little control over the dimension of a final drawn article as compared to die drawn processes.
Using filler in an oriented polymer composition is desirable both to reduce the cost of a polymer composition and also to promote cavitation. Both of these features are attractive for preparing oriented polymer compositions that can serve as alternatives to wood in structural applications such as composite decking where cost and weight are both important. Desirably, oriented polymer compositions in such structural applications are free of handicaps associated with organic fillers, density and composition instability in the presence of humidity that reactive inorganic fillers are subject to, high densities associated with inorganic filler and a low delamination force with high linear draw ratios.
An oriented polymer composition containing a large amount (thirty weight-percent or more based on polymer composition weight) of inert inorganic filler that has a density comparable to or less than wood (that is, less than 0.8 g/cm3) and strength and stiffness sufficient to meet building codes for use in structural applications is desirable. It is further of interest to have such an oriented polymer composition that is essentially free or completely free of blowing agent. It is still further desirable for such an oriented polymer composition to have a delamination force of at least 44.5 Newtons (ten pounds force) to resist delamination and fibrillation during use.
Measure the density of a polymer composition according to American Society for Testing and Materials (ASTM) method D-792-00.